This invention relates to a damper, particularly a hydraulic damper used with a weighing apparatus to damp the vibration which occurs as a result of an article being loaded onto the loading tray of the apparatus.
For today's weighing apparatuses, in addition to the demand for improvements in automation and in weighing precision, there is also a considerable demand for the weighing operations to be performed at a higher speed.
However, when an article is loaded onto the loading tray of the weighing apparatus, the impact force of the loading causes the free or movable end of the elastic member which supports the tray to vibrate substantially vertically. The vibration continues until the mechanical friction of the apparatus, the air resistance of the movable part which accompanies the vibration, etc., have caused a loss of energy and a natural damping of the vibration. Thus, if the calculation of the weight is delayed until the natural damping occurs, the weighing operation will take a long time and it will be impossible to achieve the desired high-speed weight measurement.
For this reason, in order to increase the speed of the weighing process, it is common to forcedly damp the vibration by providing a suitable type of damper between the elastic member and the frame of the apparatus. Magnetic dampers are rather expensive relative to their damping performance or effect, and they are seldom used. Pneumatic dampers are also seldom used, because the low flow resistance of the gaseous medium used in the dampers provides only a slight damping effect. Thus, the most general type of damper used on weighing apparatuses is the hydraulic type.
FIG. 10 of the accompanying drawings shows a conventional weighing apparatus having a hydraulic damper. The apparatus includes a frame F, an elastic load cell E in the form of a parallelogram fixed to the frame F, a loading tray L supported on the free end E1 of load cell E, and a piston plate P connected to the end E1 via a rod R. Also fixed to the frame F is a dashpot D containing water, oil, mercury, or some other fluid in accordance with the desired damping characteristics. The piston P is immersed in the liquid with a narrow gap G formed between the piston and the peripheral wall of dashpot D.
During the weighing process, the piston P vibrates with free end E1 of the load cell. This moves the fluid inside the dashpot D against resistance through the gap G, and the resistance effectively diminishes the amplitude of vibration.
Especially with an oil damper, the high viscosity of the oil provides a large resistance when the oil passes through the gap G, thus achieving a much larger damping effect or force than could be obtained from a pneumatic damper. By selecting an appropriate viscosity for the oil or other fluid and an appropriate size for the gap through which the fluid passes, it is possible to obtain a wide range of damping effect, and thus this type of damper features ideal performance with the optimum transition response characteristics for enabling a weighing apparatus to change from a freely vibrating state to a generally steady state generally in one to two cycles.
However, because the top of dashpot D is open, whenever the apparatus is moved, the fluid must be removed from the dashpot in order to prevent it from being spilled, thus making it very inconvenient to move the apparatus. Another problem is that dust, etc. can fall into the open dashpot D from above and become lodged in the gap G, thus causing the damping characteristics to become unstable. Also, a different type of fluid or other foreign matter can become mixed into the dashpot, changing the viscosity, etc. of the fluid and also changing the fluid level and the buoyancy, which will be discussed later, thus changing the damping characteristics.
Furthermore, because the depth to which the connecting rod R descends into the fluid changes in accordance with the weight of the articles, thus changing the volume of rod R in the fluid, the buoyancy of the rod, i.e., the upward force exerted on the free end E1 of the elastic member, changes, thus making it impossible to obtain precise measurements on weighing apparatuses which are used to measure small loads.
In addition, the surface tension of the fluid on the rod R and the peripheral wall of dashpot D also exerts an upward or downward force on the free end E1 of the load cell in the same way as the buoyancy just mentioned. Because minute changes in these forces occur as a result of the changes in surface area in contact with the fluid caused by the movement of the rod R, there is an undesirable or adverse effect on weighing precision, particularly where small loads are being weighed.